1. Field of the Invention
The present invention relates to a method for controlling exposure and a scan-exposure apparatus, which are used when a photosensitive material such as a printing plate is scan-exposed by a light beam emitted from a light source which moves along a main scanning direction or a sub-scanning direction.
2. Description of the Related Art
Some image exposure apparatuses, which expose photosensitive materials such as a photosensitive planographic printing plate (hereinafter, referred to as a “printing plate”) used in printing or the like, include light sources such as a semiconductor laser or an LED, and scan-expose the printing plate by using a light beam emitted from the light source. Further, some image exposure apparatuses wind the printing plate onto a peripheral surface of a rotating drum and rotate the rotating drum in a predetermined direction to carry out main scanning for the printing plate, and move the light source, which is disposed opposite to the peripheral surface of the rotating drum, in an axial direction of the rotating drum to carry out sub-scanning for the printing plate.
In some scan-exposure apparatuses provided in such image exposure apparatuses, a light source is mounted on an exposure stage that is disposed opposite to the peripheral surface of the rotating drum and that is movable in a sub-scanning direction (i.e., the axial direction of the rotating drum). By moving the exposure stage in the sub-scanning direction, sub-scanning is carried out. In other words, in such a scan-exposure apparatus, an exposure head that is formed by mounting a light source on an exposure stage is moved in the sub-scanning direction.
In order to increase the accuracy with which an image is formed on a printing plate in an image exposure apparatus in which a printing plate is wound onto a rotating drum and an exposure head is moved in the axial direction of the rotating drum to scan and expose the printing plate, it is necessary to focus the light beam and adjust the position irradiated thereby when the rotating drum and the exposure head are assembled with the image exposure apparatus. Further, in order to carry out highly precise adjustments of this kind, it becomes necessary to machine with high precision the parts for the scan-exposure apparatus at the time those parts are manufactured.
For example, in order to adjust the distance between the rotating drum and the exposure head, the rotating drum must be mounted at an exact position on a surface plate and a guide for guiding the exposure head in the sub-scanning direction must be abutted and fixed at a positioning piece formed on the surface plate. Thereafter, the distance between an axis of the guide and an axis of the rotating drum must be accurately adjusted by using a predetermined jig. As a result, when the exposure head is moved along the guide, the distance between the exposure head and the rotating drum remains constant.
Further, in order to adjust the position on the rotating drum irradiated with the light beam, the surface of the surface plate on which the guide is mounted needs to be accurately machined, and, for example, a shim must be nipped between the guide and the surface plate when the guide is mounted on the surface plate.
It is necessary to machine the various parts of the scan-exposure apparatus with high precision at the time those parts are manufactured and assemble them with high precision in order to accurately adjust the positions as mentioned above, whereby the production cost of the scan-exposure apparatus increases.